Additional Study Information: The purpose of this study is to assess immune recovery in patients undergoing hematopoietic celltransplantation. Stem cells are the most undeveloped cells in the bone marrow from which all the various types of blood cells are derived. These patients received stem cell transplantations for either treatment of a malignancy or to produce acceptance of an organ transplant. In order to study this, it is important to compare the patient immune response with the responses of a normal healthy individual. We may test the response of your cells to other control cells in the presence of a drug Siplizumab (anti-CD2). If the results are promising, this drug may be used in later studies to help induce tolerance in patients.